More app-friendly event description (#6684)

* More app-friendly event description

* change origin -> owner

* checked all decl_event! and changed decriptions.

* annotated parameter names for remaining events

substrate/frame/session/src/lib.rs

@@ -484,7 +484,7 @@ decl_storage! {
 
 decl_event!(
 	pub enum Event {
-		/// New session has happened. Note that the argument is the session index, not the block
+		/// New session has happened. Note that the argument is the [session_index], not the block
 		/// number as the type might suggest.
 		NewSession(SessionIndex),
 	}